This invention relates to a method of using an improved membrane to separate gas mixtures. In particular, this invention pertains to the use of a modified membrane containing fluorine or fluorine and sulfur oxide functionalities at the membrane surface which improves the gas separation properties over the unmodified membrane.
The use of membranes to separate gases is well known in the art. Gas separation membranes have been fabricated from a wide variety of polymers and incorporated into different device configurations. Such membrane devices have been used to separate oxygen, nitrogen, carbon dioxide, methane, hydrogen, and other gases from gas mixtures.
An ideal gas separation membrane possesses a high separation factor (selectivity), high gas permeability, good temperature and chemical resistance, and good mechanical strength. Polymeric materials possessing high selectivities generally have undesirably low permeabilities. Those polymers having high permeability values usually generate unacceptably low separation factors. A choice between a high gas permeability and a high separation factor has been required.
Fluoropolymer membranes have attractive gas separation factors and good chemical and temperature resistance but low, commercially uneconomical gas fluxes for several gas separations of interest. A membrane which overcomes the necessity of choosing between a high selectivity and a high gas flux has long been sought.
Surface treatment of the membrane provides one possible way to obtain high selectivity and flux combinations. Fluorination of polymers with pendant ester, carboxylic acid, acid halide, or acid anhydride groups by gaseous exposure is described in U.S. Pat. No. 4,076,916. However, the treatment method described uses disadvantageously long exposure times (days) and high F.sub.2 concentrations which may cause degradation. Surface halogenation treatment techniques have also been applied to polyolefin films used to separate liquid organic chemicals such as naphthenes and paraffins. See U.S. Pat. No. 3,062,905. The surface of the polyolefin film is often heated to increase the halogenation reaction rate. In this case, the polyolefin film is generally first irradiated to promote crosslinking to increase temperature stability prior to gaseous exposure to produce good results. The problems with these techniques include long treatment times, high treatment temperatures, or irradiation in order to achieve an adequate degree of surface halogenation. Furthermore, under these conditions, degradation of the films may result.
Membranes heretofore fabricated from polymers with high selectivities possessed low gas fluxes because the membranes could not be made sufficiently thin. The need for a gas separation membrane with both high separation factor and high flux has not been met by the present art.